During the past three years, more than thirty TcON2S2 complexes containing various amine side chains have been synthesized and evaluated as potential brain perfusion imaging agents for single photon emission computed tomography (SPECT). Biodistribution data in rats show that the initial brain uptake of the Tc-99m agents is generally good. However, the retention of the amine derivatives do not parallel the corresponding iodinated aromatic amines, IMP and HIPDM, which display good brain retention. One striking observation of our Tc-99m compounds containing amine side chains is that the steric effects appear to play a significant role on the in vivo brain uptake and retention. We have also investigated the quantitative structure-stability relationship of Tc compounds using several parameters for predicting the in vitro stability. The solid angle factor sum (SAS) and transvacancy angle (FAv) are useful for predicting stability and designing new radiopharmaceuticals based on the TcO core. This is the first time this type of quantitative stability relationship has been explored Tc complexes. These types of studies will enhance our ability to design better Tc-complexes with predictable in vitro and in vivo stabilities and facilitate the development of second generation Tc agents targeted to a specific chemical or enzymatic process (es). Currently, more than 85% of routine clinical procedures involve Tc-99m; the future of nuclear medicine depends on new Tc compounds. Favorable cost to benefit ratio for investments in the development of new Tc-99m imaging agents makes this project a worthy endeavor. In the next funding period, we propose to prepare a group of second generation Tc-99m brain imaging agents are designed to form neutral and lipid soluble complexes with TcO(III). The ester or amide derivatives will be hydrolyzed by a non-specific pH-dependent hydrolysis or by specific enzymatic (peptidase) hydrolysis. The built-in fluorine atom(s) can be employed for fine tuning the rate of hydrolysis. These mechanisms will permit the initial investigation of new trapping mechanisms to aid in the development of new brain imaging agents aimed at changes in brain perfusion or metabolism due to diseases such as stroke or brain tumor. It is possible that the agent will be measuring flow directly or a combination of hydrolysis and a specific or non-specific process(es) related to the disease stages.